1. Field of the Invention
The present invention relates to electrolessly plated nickel coatings and machine parts utilizing the coatings. More precisely, it relates to plated coatings showing high impact strength, high hardness and improved lubricating characteristics such as sliding characteristics (friction coefficient, wear resistance, anti-seizure property).
2. Related Art
Electroless plating is more advantageous than electrolytic plating in terms of the fact that it enables to easily produce uniform plated coatings only by immersing articles to be plated into plating baths. Electrolessly plated coatings composed of nickel added with phosphorous and/or boron have been utilized as surface treatments of various machine parts and the like since they shows relatively high hardness (Japanese Patent Un-examined Publication (A-1 publication) Nos. 51-109224 (109224/76) and 61-291979 (291979/86)). Such coatings further containing microparticles of nitrides, carbides or the like have also been known (Japanese Patent Un-examined Publication No. 4-329896 (329896/92)).
Electrolessly plated coatings composed of nickel and phosphorus (Ni--P) may be advantageously produced at a high rate since plating baths for such coatings are generally stable. In order to obtain such coatings with high hardness, heat treatments at a high temperature, i.e., more than 280.degree. C., are required. However, such heat treatments at a high temperature may reduce hardness of base materials composed of, for example, precipitation hardening type aluminum alloys and simultaneously reduce toughness of the coatings themselves and hence their impact strength may be also reduced.
Electrolessly plated coatings composed of nickel and boron (Ni--B) have a problem that it is difficult to deposit such coatings at a high rate since plating baths for such coatings are unstable. On the other hand, these coatings have an advantage that they can have relatively high hardness (HV 650 to 750) with heat treatments at a temperature around 200.degree. C. However, to obtain a higher hardness exceeding HV 800, they requires heat treatments at a further higher temperature and, in such a case, they may show the same problem as the Ni--P coatings.
Electrolessly plated coatings composed of nickel, phosphorus and boron (Ni--P--B) having the following compositions have also been known:
1 P: 6.8 to 7.8% by weight, B: 0.7 to 0.3% by weight, PA1 2 P: 1.4 to 6.6% by weight, B: 2.1 to 7.1% by weight, and PA1 3 P: 0.3 to 0.5% by weight, B: 4.7 to 9.8% by weight.
Coatings having a composition of 1 have a high phosphorus content and show their properties similar to those of the Ni--P coatings and they cannot acquire sufficient hardness by heat treatment at a low temperature. Coatings having a composition of 2 are formed by deposition from acidic baths containing NaBH.sub.4. Such baths are very likely to decompose and cannot be used in practical applications. In addition, experimentally produced coatings having such a composition have shown a hardness of only HV 672 at most. Coatings having a composition of 3 are formed by deposition from alkaline baths containing NaBH.sub.4. Since such baths are highly alkaline, they may corrode base materials composed of aluminum alloys or the like and they cannot be formed on the base materials without protective layers. If such protective layers are provided, they constitute intermediate layers between the plated coatings and the base materials, but existence of such intermediate layers may disadvantageously reduce their hardness.
Therefore, an object of the present invention is to provide electrolessly plated coatings which can be suitably provided on base materials composed of aluminum alloys or the like, show high hardness, high impact strength (high toughness) and improved lubricating characteristics such as sliding characteristics and, in addition, can be formed at a high rate.
Another object of the present invention is to provide sliding machine parts of which sliding surfaces are covered by the plated coatings described above and show high impact strength (high toughness), high hardness and improved lubricating characteristics such as sliding characteristics.